1. Field of the Invention
The present invention relates to telecommunications systems and more particularly to determining the location of a telephone in order to facilitate 911 service or other location-based services.
2. Description of Related Art
For many years, the public switched telephone network and 911 emergency service centers have been equipped to determine the location of emergency callers, in order to facilitate emergency response.
Until recently, such calls originated mainly from landline telephones. Therefore, the process of determining the location of a caller was a usually a simple matter of looking up the street address of the calling telephone number. For instance, when an emergency services network (ESN) would receive a call setup message reflecting an incoming call, the ESN could read the calling number from the setup message and then dip into an Automatic Location Identification (ALI) database that correlates telephone numbers with street addresses, in order to determine the street address of the caller. The ESN could then deliver that address to an emergency services operator, to assist in handling the call.
With the advent of cellular telephony, this sort of simple database lookup became insufficient. Unlike landline telephones that were fixedly associated with particular street addresses, cellular telephones could place calls from anywhere within a cellular coverage area. Thus, a more complex system of position determination was required in order to determine the location of a calling cellular telephone.
To facilitate 911 services for cellular telephone callers, the United States Federal Communication Commission (FCC) mandated the implementation of “Enhanced 911” (“E911”) services.
The E911 mandate was divided into two phases. According to Phase 1, the location had to be identified with an accuracy of at least cell and sector. As this information is typically maintained by a cellular wireless carrier in a subscriber's home location register (“HLR”), Phase 1 presented little technical challenge. According to Phase 2, the location must be provided with an accuracy of at least 100 meters (or 50 meters for handset-originated methods such as GPS), which is far more granular than the cell and sector information maintained in the HLR. In response, the Telecommunications Industry Association (TIA) has proposed a standard entitled “Enhanced Wireless 9-1-1 Phase 2” or “J-STD-036-A” (including a recent addendum, J-STD-036-A-1), the entirety of which is hereby incorporated by reference.
In order to achieve the accuracy specified by Phase 2, a cellular wireless carrier may employ a wireless position determining system that communicates with the cellular telephone in order to determine its current location. The positioning system could be “handset-based,” in which case the cellular telephone would read its geographic coordinates from a satellite based positioning system (e.g., GPS) and report the coordinates over an air interface to the carrier. Alternatively, the positioning system could be “network-based,” in which case the carrier might employ a technique such as triangulation (or, more specifically, “advanced forward link trilateration” (AFLT)) to measure the telephone's location. Alternatively, the position system could employ a combination of handset and network-based techniques.
In current practice, when a person places a 911 call from a cell phone, the wireless carrier can begin the wireless position determining process and send a key code (call identifier) to the ESN in a call setup message. Upon receipt of the call setup message, the ESN may then send ask the wireless carrier for the location that corresponds to the key code. In response, the carrier may then match the key code to the location that it determined by the wireless position determining process and may then report that location to the ESN.
An ESN distinguishes between landline and wireless (cellular) callers by the trunks on which the calls arrive at the ESN. In particular, the ESN would receive landline calls on trunks from landline switches (switches known to serve landline callers), and the ESN would receive wireless calls on trunks from wireless switches (switches known to serve wireless callers).
Thus, when the ESN receives a call setup message, it conventionally takes a location-determination action based on the trunk on which the call is being delivered. If the call is arriving on a trunk from a landline switch, the ESN dips into an ALI database to map the calling number to a street address. On the other hand, if the call is arriving on a trunk from a wireless switch, the ESN queries the wireless carrier for the caller's location.